Anniverseries that aren't

I have several newlywed friends who are into celebrating at the drop of a hat, and as such have thrown parties for their “1 month anniversary”, “3 month anniversary”, etc. I also know that according to my Webster’s New Collegiate Dictionary, anniversary means “the annual recurrence of a date marking a notable event”. So obviously a 1 month anniversary makes about as much sense as celebrating a 1 month millenium. My question thus becomes:
What do you call the periodic but not necessarily annual recurance of a date marking a notable event?

Um, how about “anniversary”? Although anniversary originally meant “day recurring yearly”, the usage of “three -month anniversary” has become fairly common. I expect that this usage will makes its way into the dictionaries before too long.

The Mavens’ Word of the Day has a column on this about a year ago. Basically, it talked about how we have borrowed the word “anniversary” from ourselves to fit a need for which we didn’t have a word. Our language is flexible, and just as the meaning of “decimated” has changed (at least in common usage), the meaning of “anniversary” is changing.

See an earlier thread: Monthly wedding remembrance. If you absolutely must celebrate them, you should call them mensiversaries.